Recent developments in grain refinement have included various non-chemical means for refining alloys. These non-chemical means include transmitting ultrasonic waves through the melt and electromagnetic and/or mechanical stirring of the melt. In each of these non-chemical means the liquid alloy is agitated to break up dendrites that would normally form in the melt during casting.
U.S. Pat. No. 3,902,544 issued to Flemings et al discloses a process for making solid or liquid-solid metal composition containing non-dendritic primary solids and to the process for shaping such compositions. The process includes using mechanical stirrers to agitate the liquid metal prior to casting.
U.S. Pat. No. 4,315,538 issued to Nielsen discloses a method and apparatus for continuous casting of alloys for obtaining a fine grain size therein. The improved arrangement comprises a means associated with the die for causing uniform temperatures in the liquid alloy throughout the near freezing area of the die. These means prevent the presence or development of any thermal gradients which are large enough to produce gross directional solidification of the alloy. The means includes focusing the entry of the alloy into the die in a manner which imparts a generally cyclonic motion to the liquid alloy. Nielsen discloses that prior art methods had a smallest grain size normal to casting of 0.063 inch (1500 microns) and that his method produced grain sizes as low as 0.031 inch (750 microns).
U.S. Pat. No. 4,373,950 issued to Shingu et al discloses a process of preparing aluminum of high purity. The aluminum is purified by using a mechanical stirrer to break down dendrites extending from the liquid-solid interface into the liquid phase to release impurities from between the dendrites or between the branches of the dendrites, and dispersing the released impurities throughout the entire body of the liquid phase.
Heretofore, stirrers have not been entirely successful for direct chill casting of ingots. Stirrers create a turbulence in the melt surface which may entrap air and/or surface oxides and pick up hydrogen from the atmosphere. These entrapments lower the integrity and quality of the cast ingot. The manufacturing of semifinished metallurgical cast products such as ingots, billets and plates must be such that the cast products have the best possible degree of physical and chemical homogeneity. This is necessary in order to avoid the occurrence of certain defects in the subsequent operation of transforming such products into other shapes such as sheets and wires.
It would be advantageous, therefore, to provide a method and apparatus that can be readily added to existing casting facilities which permits the use of mechanical stirrers in the direct chill casting of ingots without creating entrapments in the cast ingot.
The principal object of the present invention is to provide a low-cost process for producing high quality alloy ingots with a fine grain size.
Another object of the present invention is to provide a process for producing high quality alloy ingots that do not suffer from the disadvantages of the prior art.
Yet another object of the present invention is to provide a process for producing high quality alloy ingots having refined grain structure which are relatively inexpensive to manufacture.
Still another object of the present invention is to provide a process for producing high quality alloy ingots having refined grain structure which does not require the preliminary addition of refining agents.
Another object of the present invention is to provide a process for producing high quality alloy ingots that can be readily used in existing direct chill casting facilities.
These and other objects and advantages will be more fully understood and appreciated with reference to the following description.